Volumio AirPlay quality

Hey all,

I have a question about the quality of AirPlay. When I play the same MP3 song on my iPhone via AirPlay and local on the Volumio device, the quality highly differ.

I wonder why. Is there maybe something wrong with my network or settings? I am clearly able to hear a difference. When played with AirPlay the song quality is bad compared to when I play it local on the Volumio device.

Can someone tell me why? And can I improve it somehow to be better quality when played with AirPlay? I thought playing songs with AirPlay is like having CD-Quality. I read that AirPlay is better than bluetooth.